Since a xenon lamp has spectroscopic properties approximately the same as those of solar rays, it is especially suitable as a light source for a light fastness tester and has come into wide use in recent years.
An example of such a tester is illustrated in FIG. 1. The xenon lamp 2 is vertically disposed at the center of a testing tank 1, and a sample frame 4 having a sample or samples 3 aligned and supported circumferentially around and spaced from the xenon lamp 2 at the center thereof. The sample frame is rotatable on a rotary shaft 5 mounted in bearing 6, and is driven through a bevel gear 7 by a motor 8. While being thus rotated, the sample 3 is irradiated by the rays of light emitted by the xenon lamp whereby the aging, discoloring and fading of the samples are produced. The air temperature inside the tank 1 is elevated due to the heat generated by the lamp 2 during light emission. However, a blower 10 controlled by a temperature controller draws in air from outside the tester and opens an exhaust valve 11 that is opened and closed in accordance with the air pressure produced by the blower 10, thereby replacing the air in the tester with external air and thus automatically maintaining the air temperature inside the tank 1 at a constant level.
In this apparatus, the xenon lamp 2 acting as the light source, which plays the most important role in the light fastness tester, is operated to produce a stable emission of light by means of a lighting device 12. In order to prevent breakage of the lamp 2 due to the high exothermy during light emission, the circumference of the lamp is cooled with water.
The construction of the conventional means for water-cooling the xenon lamp will now be explained with reference to FIG. 2. The xenon lamp 2 is surrounded by a cylindrical inner tube 13 made of an ultraviolet-transmissive glass and the outer circumference of the inner tube 13 is further surrounded by an outer tube 14 made of a similar material. The cooling water enters from an inlet tube 15, flows downward between the xenon lamp 2 and the inner tube 13 while cooling the surface of the lamp, and next flows upward between the inner tube 13 and the outer tube 14 and is finally discharged from an outlet tube 16.
The xenon lamp 2 has a high voltage terminal 17 at the lower end and a low voltage terminal 18 at its upper end, a high voltage lead wire 19 and a low voltage lead wire 20 being connected to the high and low voltage terminals respectively. The other ends of these wires 19 and 20 are connected to the aforementioned lighting device 12. Due to the structural arrangement of the light fastness tester 1 and the means for cooling the xenon lamp, however, the high voltage lead wire 19 extends through the testing tank between the xenon lamp 2 and the sample and, hence, is exposed to the emitted rays of light. In addition, during the time the lamp is lighted, the lead wire 19 is carrying a voltage of several kilovolts or more.
Xenon lamps themselves have conventionally been used in great quantities for advertising and general illumination purposes but only in an extremely limited quantity for the light fastness testing purposes. When a xenon lamp is used for ordinary illumination, the energy in the visible range becomes important in determining the quality of the lamp, whereas the quantity of energy in the ultraviolet range and the drastic attenuation of the energy in the ultraviolet range, which is inherent in discharge lamps of this kind, does not pose a problem. On the contrary, in practice the lamp is used in combination with a filter to cut off the rays in the ultraviolet range.
For light fastness testing purposes, however, research and development have thus far been directed to solving the problems and improving the properties of xenon lamps such as reduction of transmissivity of ultraviolet rays, which is an essential property of the lamp if it is to be used for such testing, due to the structure of the water-cooling means for the lamp, attentuation of the ultraviolet rays during long-running tests (500-2,000 hours) and damage of the xenon lamp. As a result, a xenon lamp having water-cooling means as described above with reference to FIGS. 1 and 2 (hereinafter referred to as a "xenon lamp device") has been developed and is now in practical use.
However, the xenon lamp device shown in FIG. 2 does not provide sufficient stability of the test conditions for the purpose of the light fastness tests. Namely, the conventional xenon lamp water-cooling means shown in FIG. 2 has the following drawbacks:
(1) Heat exchange efficiency is poor during cooling of the lamp.
The cooling water flows from the top to the bottom between the lamp 2 and the inner tube 13 while cooling the surface of the xenon lamp 2, that is, while the temperature of the cooling water is rising, and the thus heated water then flows from the bottom to the top between the inner tube 13 and the outer tube 14. In this arrangement, transfer of heat occurs from the upwardly flowing cooling water, which is at a higher temperature, outside the inner tube to the downwardly flowing cooling water, which is at a lower temperature, inside the inner tube, thereby reducing the cooling efficiency.
(2) Pressure loss is great during the flow of the cooling water.
Since the cooling means has the double tube construction wherein both the inlet and outlet for the cooling water are at the upper portion of the means and the tubes are constructed so that the flow passage is bent double at the lower end to direct the downwardly flowing cooling liquid into the return passage, the frictional resistance at the surface of the tubes is greater by about three times than for a straight through flow cooling water tube. Since the temperature of the cooling water in the layer contacting the surface of the high temperature xenon lamp is raised, the specific gravity of the water in said layer becomes less and the water tends to rise upwardly against the downward flow of the main stream of the fluid. The same reverse flow phenomenon takes place, although to a slightly less degree, in the water layer at the outer surface of the inner tube 13 due to the aforementioned transfer of heat. Furthermore, air bubbles tend to form in said layer against the high temperature surface of the lamp and the air bubbles thus formed rise upward against the flow. These phenomena together result in a great pressure loss. Therefore, if the water main pressure of the cooling water being fed is low, the quantity required for cooling will not flow through the means and a separate pump becomes necessary for raising the pressure, thus resulting in the need for additional equipment.
(3) Loss of irraiation energy (especially attenuation of the ultraviolet rays) directed toward the sample 3 is great.
Contamination of both surfaces of the inner tube 13 and the inner surface of the outer tube 14 (three surfaces all together) resulting from the quality of the water is great and is three times as great as on the single surface of a single tube. The water layer through which the light is transmitted is thick, being double that of a single tube. For these reasons, the transmissivity of the light is low, and attenuation, over the course of time, of the energy of the ultraviolet rays having an especially short wavelength becomes a critical problem in a long-running light fastness test such as a continuous test for 50-2,000 hours.
(4) Degradation of the high voltage lead wire occurs.
As initially explained with reference to FIG. 1, the high voltage lead wire 13 (and the low voltage wire in certain cases) must pass through the gap between the sample 3 and the xenon lamp 2 because of the arrangement of the xenon lamp device secured to the upper portion of the tester and suspended therefrom, and the sample frame 4 rotatably supported below the lamp device. Consequently, the coating material of the lead wire is degraded in the same way as the sample 3 and its service life is shortened.
(5) The lead wire shades the sample being tested. It is not desirable that the lead wire constantly shade the sample 3, even slightly. This shading particularly disturbs the control of the light by an automatic irradiation energy controller incorporating a sensor sensitive to the light and disposed at the position of the sample.